This invention relates generally to suspension systems and methods for isolating and reducing the transmission of vibratory motion between an object and a base and, more particularly, to a passive means for eliminating or substantially reducing the sensitivity of the axial position and natural frequency of a negative-stiffness vibration isolator to changes in ambient temperature.
The problems caused by unwanted vibration on equipment, devices and processes that are extremely motion sensitive have been widely researched and numerous solutions to prevent or reduce the transmission of vibration motion have been proposed and developed. Many of the devices designed to reduce the transmission of unwanted vibration between an object and its surroundings, commonly called vibration isolators or suspension devices, have utilized various combinations of elements including resilient pads made from a variety of materials, various types of mechanical springs, and pneumatic devices. There are, however, shortcomings and disadvantages associated with these particular prior art isolation systems which prevent them from obtaining low system natural frequencies and from limiting resonant responses to low values while providing high isolation performance at higher frequencies.
These shortcomings and disadvantages of prior art systems were addressed through the development of novel vibration isolation systems, devices, and methods for retrofitting existing vibrating systems, described in my U.S. Pat. No. 5,530,157, entitled “Vibration Isolation System” issued May 10, 1994; U.S. Pat. No. 5,370,352, entitled “Damped Vibration System” issued Dec. 6, 1994; U.S. Pat. No. 5,178,357, entitled “Vibration Isolation System” issued Jan. 12, 1993; U.S. Pat. No. 5,549,270, entitled “Vibration Isolation System” issued Aug. 27, 1996; U.S. Pat. No. 5,669,594, entitled “Vibration Isolation System” issued on Sep. 23, 1997; and U.S. Pat. No. 5,833,204, entitled “Radial Flexures, Beam-Columns and Tilt Isolation for a Vibration Isolation System” issued on Nov. 10, 1998, which are all hereby incorporated by reference in their entirety in this present application.
The particular vibration isolation systems described in the above-identified patents, and which could be utilized in connection with the present invention, provide versatile vibration isolation by exhibiting low stiffness in an axial direction (generally the direction of the payload weight) and any direction substantially transverse to the axial direction (generally a horizontal direction). The vibration isolation system may include a tilt-motion isolator for providing isolation in all six degrees of freedom, three translations and three rotations. The particular systems utilize a combination of unidirectional or bidirectional isolation subassemblies that can be connected together in series fashion to provide omnidirectional isolation. Each isolator is designed to isolate either the axial or the transverse component of any vibratory translation to effectively isolate vibrations along, or about, any directional axes. In subsequent discussions, an axial-motion isolator may also be referred to as a vertical-motion isolator, and the system of axial-motion isolators may also be referred to as a vertical-motion isolation system. Similarly, a transverse motion isolator may be referred to as a horizontal-motion isolator, and the system of transverse-motion isolators may be referred to as a horizontal-motion isolation system. Lastly, a tilt-motion isolator which comprises a mechanism which allows rotations about the tilt axes (pitch and roll) may be referred to as a tilt-motion isolation system.
In the embodiments described in the above-noted patents, the isolator system relies on a particular principle of loading an elastic structure which forms the isolator or a portion of it (the loading being applied by either the supported weight or by an external loading mechanism) to approach the point of elastic instability of the elastic structure. This loading of the structure to approach this point of elastic instability, also referred to as the “critical buckling load” of the structure, causes a substantial reduction of either the vertical or horizontal stiffness of the composite isolator to create an isolation system that has low stiffness in the vertical direction and in any horizontal direction, and increases the damping inherent in the structure. While stiffness is greatly reduced, the isolator still retains the ability to support the payload weight.
In the event that the load on the elastic structure is greater than the critical buckling load, the excessive load will tend to propel the structure into its buckled shape, creating a “negative-stiffness” or “negative-spring-rate” mechanism. By combining a negative-stiffness mechanism with a support spring, adjusted so that the negative stiffness cancels or nearly cancels the positive stiffness of the spring, one obtains a device that can be placed at or near its point of elastic instability. The magnitude of the load causing the negative stiffness can be adjusted, creating an isolator that can be “fine tuned” to the particular stiffness desired.
These above-described isolators provide excellent systems for isolating or reducing the transmission of vibratory motion between an object and the base by exhibiting low stiffness, high damping to limit resonant responses of the system, effective isolation at high frequencies and higher isolator resonant frequencies, while being capable of accommodating changing weight loads without significantly degrading isolation system performance. Passive negative-stiffness vibration isolators like the ones described in my above-identified patents provide very low vertical and horizontal natural frequencies, typically 0.5 Hz or less, and thereby provide exceptional vibration isolation at low frequencies and over a range of vibration frequencies encountered in buildings, floors and ground locations, typically in the range of 1 to 80 Hz and higher. Because of the use of negative stiffness to achieve the low vertical stiffness and vertical natural frequency, these isolators are sensitive to changes in temperature that can cause a change in vertical position and that also can result in a change in vertical natural frequency of the isolator. The lowest vertical natural frequencies are achieved when the isolator is at an optimum vertical equilibrium position and, because of nonlinear effects, the frequencies increase as the vertical position changes from this optimum position. Accordingly, these isolators may require manual adjustments, from time to time, following small changes in the weight load on the vertical-motion isolator, or dimensional changes to the structure of the isolator caused by creep of the main support spring or variations in ambient temperature.
Changes in the ambient temperature to which the isolator is subjected to can produce changes in the optimum equilibrium position due to resulting changes in the dimensions of the isolator springs, flexures, and other structures from relative thermal expansions and from changes in mechanical properties such as elastic modulus. Generally, the lower the natural frequency the greater the sensitivity of vertical position and frequency to changes in temperature. Negative-stiffness isolators have simple manual adjustments for correcting for changes in vertical position and some negative-stiffness isolators utilize electro-mechanical auto-adjust systems for sensing changes in vertical position and automatically correcting for them. Therefore, an adjustment apparatus which can continuously and automatically adjust the axial-motion isolator of such an isolation system in response to such variations would further enhance the already high performance of such vibration isolation systems.
My U.S. Pat. No. 5,794,909, incorporated herein in its entirety by reference, discloses such an electro-mechanical automatic adjustment apparatus which virtually eliminates, or greatly reduces, the need for manual adjustment of the isolator. The apparatus disclosed in U.S. Pat. No. 5,794,909 is an active system which requires electronic sensors to sense any change in the isolator position and electronic controls to apply a signal to a component which adjusts the load on a compressed control spring which moves the isolator back to its optimum equilibrium position. However, there are situations where the isolators are not accessible or easily accessible, as when the isolator is placed in vacuum chambers, inside acoustic enclosures, or at remote locations. In some applications, very tight tolerances on isolator vertical position and natural frequency are desirable and sometimes it is not practical to make frequent manual adjustments, particularly in multiple-isolator systems. Too many manual adjustments also could cause dynamic perturbations in the isolation system that would be detrimental to the particular application. Because of these situations, it is desirable to have a passive means for automatically compensating for the inherent temperature sensitivity of negative-stiffness isolators. The present invention satisfies these and other needs.